This invention pertains generally to vapor deposition of films on substrates and more particularly to a reactant gas injector apparatus and process of improving uniformity of thickness and resistivity of deposited films in chemical vapor deposition reactors.
Heretofore, silicon compounds have been deposited on semiconductor wafers and other substrates in high temperature oxide (HTO) reactors which commonly include an elongated tube defining a reaction chamber and a heating source surrounding the tube for heating wafers in the chamber. Reactant gases flow through the chamber or plenum in an axial direction, and the wafers are positioned with their major surfaces perpendicular to the direction of gas flow.
Increased sophistication and density of integrated circuit devices demands tight uniformity specifications which chemical vapor deposition (CVD) process equipment must satisfy. As the diameter of semiconductor wafers has increased and the requirement for processing more wafers per batch has accelerated, it has become increasingly difficult to maintain sufficient control over uniformity of film thickness. For example, if between 75 and 100 four-inch wafers per batch are being processed, it is not only difficult to maintain uniformity of film thickness across the area of each wafer but also from wafer to wafer in each batch.
Reactors have been modified in the past to improve uniformity of film thickness across individual wafers and across the wafer boat. For example, a perforated distribution tube positioned within a horizontal reaction tube has been used to redirect gas flow patterns.
Vertical reactors possess certain advantages over their horizontal counterparts. Gases are exhausted only from the bottom, and those gases inside the tube can only be pushed out by gases entering from the top. As a result, nearly perfect plug flow can be achieved. A vertical configuration also provides for precision and reproducibility in both the horizontal and radial placement of the carrier within the tube. In some vertical reactors, some reactant gas preheating and premixing occurs. However, improved uniformity in film thickness is achieved through, for example, changing the susceptor thickness, which permits control of induced heating at prearranged susceptor locations, altering the temperature distribution within the susceptor and wafers; reducing the thickness of susceptor supports for greater heating at the edges of susceptors which are susceptible to greater heat loss; and boundary control arrangements which alter gas flows, so that all reactant gases are not deposited on the first wafers to contact the gases. All of these mechanical changes require substantial equipment modification.
Variations in the vertical design to improve uniformity of film thickness have been tried, with limited success. For example, some improvement in film thickness uniformity is achieved by adjusting the height-to-diameter ratio of the bell jar along with controlling susceptor temperature. However, attempts to alter the temperature of susceptors, increases film thickness uniformity at the expense of film resistivity uniformity.
Vertical reactors produced by Silicon Valley Group, Inc., San Jose, Calif., are another variation in the vertical reactor design. In one version, as shown in FIG. 1, the vertical furnace 3 has heating means 5, a reactant gas inlet 6, which connects gas sources 8 via a manifold 9 to reaction chamber or plenum 13. Reactant gases pass across a boat 15 of wafers 18 on which a film is to be deposited. As in typical vertical reactors, boat 15 is enclosed within reactor tube 27, having thermowell 21, exhaust 24, and a quartz pedestal 28 on which boat 15 sits. Process tube door 29 allows access to the reactor, and the whole apparatus sits on base plate 30. The film is conventionally deposited by the injection of two or more reactant gases such as nitrous oxide (N.sub.2 O) and dichlorosilane (Si.sub.2 H.sub.2 Cl.sub.2) into the lower pressure (roughly 450 millitorrs) reaction chamber 13. The rate of reaction and hence the deposition rate is a function of the process temperature, process pressure and gas flow rates. The film thickness uniformity across a wafer and across the load is a function of inter-wafer spacing, the process pressure, and the range or skew of temperatures used in the reactor.